Lots of people microwave Christmas puddings these days. For many stepfamilies, though, the traditional pressure cooker may be a more fitting symbol for the season of goodwill. Beneath the fixed lid of festive bonhomie boils a rich mix of emotions, which can generate clouds of obscuring steam. I was inside such a cloud at roughly 3pm on Christmas Day last year when, en route to my sister-in-law's, I drove into an unmarked traffic island, leaving our grot-lined people carrier with two flat tyres, my mother-in-law, my grown-up daughter, my wife and our three young children needing a taxi urgently, and me on the phone to the AA.

I'd been thinking about my other kids, you see: two lovely mid-teenage boys - my grown-up daughter's brothers - who'd joined the rest of us that morning but by then had returned to their mother and their other home. It had been a joy to see them, not least for their half-siblings, who notice their absences constantly. Before that, though, had come churning anxiety. We'd expected them at 11am yet by half past they had failed to arrive. The little ones sat at the windows. My wife and I paced among the remaining piles of presents. Had something happened? Dare we phone? Eventually, they came. But to the toll taken by all that fretting was then added the poignancy of their leaving. I was an accident waiting to happen - quite literally.

Any type of family can have its stress points exposed by Christmas, no matter how nuclear its structure. With stepfamilies, though, as with so much else, the complications have a way of being that bit more complex, the diplomacy demanded a degree more delicate. No wonder, then, that they produce yuletide tales of such barmy extremity, especially when the fractured parents don't get on.

On the website Mumsnet, a mother using the name Carmenere tells of her then-new partner being barred by his ex from entering their once-shared house to see their kids - she had her boyfriend round, you see. For their part, the kids wanted to see their dad but not in his new home because, that's right, his girlfriend would be there. This seems not to have been personal, exactly: it just felt disloyal to their mum. "Eventually," writes Carmenere, "I agreed to stay in the bedroom so they could visit and exchange gifts with their dad."

Was it simply because they didn't like her? Or might the story also contain a theme common to many step-families at Christmas: children's desire for a relationship's broken pieces to reassembled, if only for a day? In Carmenere's stepchildren's case, the latter was impossible, but maybe the illusion that their dad hadn't found somebody else suited them better than the reality. In a different way, this need for a facsimile of former unity was apparent too in a girl called Ava, after her parents, Joanna and Matthew, split up in 1997. Although she was only 18 months old at the time, it soon became clear that Ava missed their being together. Though far from on good terms, they did their best to patch things up for Christmas Day.

"I'd told Matthew when he moved out that he'd never have Ava at Christmas," Joanna recalls. "But in the end we tried to make it work, regardless." Her in-laws played host that year. "They were so good about it. It was incredibly difficult, with me and Matthew not talking and sitting in different rooms most of the time. But it seemed all right for Ava, so we muddled on like that for the next three or four years." This solution, though, ceased to be viable after Joanna and Matthew found new partners and Ava, as a result, acquired not only a stepmum and a stepdad but also an assortment of step-relatives attached to each. Christmas visiting options and obligations multiplied. Now where would Ava be on Christmas Day? With her mum, with her dad or would it be split between them in some way?

By then, she had more say in the decision. As Matthew explains, "The older she's got, the more important her opinion has become." The dilemma was resolved collectively, resulting in Ava spending one whole Christmas Day with Matthew, the next with Joanna and so on. That arrangement still holds, and Ava, now 11, will wake up at her dad and stepmum's place on Christmas morning this year and not see Joanna until Boxing Day at the earliest: this despite the fact that doing so will mean not being with her new baby sister, Lois, on her first ever Christmas Day.

"I was quite surprised when she decided she'd be with us anyway," says Matthew. "I thought that because of Lois she might want to be at Joanna's this year and maybe it's partly because she feels she ought to be fair to me. But we've talked about that and she seems happy, which is the most important thing, obviously." And Joanna? "I'll miss Ava, of course, but I know she'll be having a great time. And after all those really horrible Christmases, it's such a relief that Matthew and I can make friendly arrangements and everything's calmed down."

For others, such settlements remain elusive; the pressure-cooker lid, while loosened by the passage of time, stays in place. Among the less heart-warming Christmas traditions is that of intact families blowing apart under the strain of having to pretend to be happy. Rowan, now in her 30s, will always associate Christmas with her father walking out on Boxing Day. She was 11 then, and didn't spend Christmas with him again until she was 18.

'People didn't do civilised divorces in those days," she explains, and Rowan had little, if any, contact with her father at all until, when in the sixth form, she started phoning him secretly. Then, after falling out very badly with her mother, she became free to visit him too. Now married and herself a mother, she spends every Christmas Day at her father's home, travelling a long distance to be with him, her stepmother and a varied cast of in-laws, step-thises-and-thats and, the way Rowan tells it, anyone else who happens to wander in. "There's usually at least a dozen people there and I'm not even related to half of them."

Since reuniting with her father, Rowan has only once done Christmas at her mum's, when her pregnancy made the journey to her dad's too daunting. It was all perfectly pleasant and polite and yet unsatisfactory in telling ways. "By the time we arrived, they'd already opened all the presents. At my dad's, we don't do that until the afternoon!"

Most families have Christmas conventions, rituals and orders of events that children especially like to honour: carrots for Rudolph, favourite tree ornaments to hang, ghastly table decorations to construct out of glitter and papier-mache. Divorce or separation and subsequent re-partnering disturb, if not destroy, many of these, and founding new ones can be tricky. If you, your children and your new partner are to be joined by his or her children from a previous life, how do you ensure that everyone gets on and no one feels left out? Part of the trick may be to have low expectations. "Begin by accepting it may not be a happy time," writes the family counsellor Suzie Hayman. "Simply by acknowledging they have sad feelings about the day and have a right to these can make a tremendous difference." And if you are a new step-parent in your new partner's children's lives, how do you fit in to an already established pattern without becoming in the process a reminder of what those children are missing?

David, a senior secondary-school teacher, was in exactly such a position when, in 1995, he spent his first family Christmas with his wife, Jane. She had given birth to their first child together, a daughter, only a few weeks earlier. Jane already had an older daughter, Ellie, whom she'd been caring for alone for the previous eight years after she and Ellie's father split up. Following years of little contact, Ellie, by then aged 10, had begun to want to see her father more often. They'd spent a little time together, and with his presence looming ever larger in her life, she felt his absence the more strongly on Christmas Day. This meant that David, having just become a father, was also a stepfather who couldn't begin to fill the dad-shaped space in Ellie's life. A whole new model Christmas had to be painfully constructed.

"It wasn't easy," David recalls. "My daughter was only four weeks old, and Ellie was having to get used to that. For the previous eight years it had been just her and Jane, and now there was this new baby, and there was me. I was very concerned to make things as easy and nice as possible for Ellie and everyone else, but it was very difficult. I think Ellie liked the idea of being part of a complete family, if you see what I mean: one with two parents and more than one child. But at the same time she couldn't help but miss her dad, even though she barely knew him and, of course, Christmas makes those feelings very strong."

As Ellie herself acknowledges, her father hadn't been doing a great job of building a relationship with her, and any hopes of that improving were dashed when he became ill and died during the ensuing year. Grief, then, overshadowed David and Ellie's second Christmas as stepfather and stepdaughter. "It was inevitable that a child in her position would have divided loyalties," David says. "And her dad's death was bound to make them deeper. It made it even surer that I couldn't ever replace him. I just tried to be stalwart and steady."

That's been David's policy ever since and, several Christmases later, it seems to have paid off. He and Jane have had a second child together, and Ellie, now 22, has a baby daughter of her own. She says she's never been happier and enthuses about a Christmas tradition that David has introduced: grown-ups opening presents on the night of Christmas Eve rather than waiting patiently till the next morning. "That's very popular with all of us," Ellie says, "especially me."

David describes their relationship glowingly: "It's brilliant. I think having a child has helped Ellie to understand all these things about families much better. We're all really looking forward to Christmas this year."

One of the few good things about family breakdown is that it can, with luck and over time, broaden and concentrate the mind: broaden its perception of whom a family can valuably include; concentrate it on the need to put the wellbeing of children first. Christmas magnifies the warmth and the fault lines alike in families of every kind, but often does so in stepfamilies with particular power. At its best, it nurtures forgiveness, selflessness and generosity - rather as Christmas itself is meant to. At its worst, it does the opposite. All over the country, negotiations will, even now, be taking place.

As for my own family's Christmas, that ought to be less of a pressure-cooker affair than the last. It will help that my younger children have asked to have the whole of December 25 at our own home, just for a change. It's nice to visit aunts, uncles and cousins on Christmas Day but, they say, it will also be nice to have more time to enjoy their presents and to spend time in the company of those much-loved big brothers and sister, who have another family to be a part of too. And, of course, it will mean that Daddy won't be driving into any traffic islands this year.

· Dave Hill's new novel, The Adoption, is published by Review on December 4